This invention relates to a method and apparatus for sampling fluids from a wellbore, and in particular to a method and apparatus used to recover a quantity of production fluids such as produced oil, gas and/or water from the wellhead of an underwater well.
Wells for hydrocarbons and other valuable fluids are normally drilled in a cluster with a number of wellbores having their surface wellheads grouped together. The wellbore may diverge away from each other the deeper they become. The wellheads in a group of wells are typically connected to a manifold or other subsea structure via conduits, and the hydrocarbons recovered from each individual well are conveyed along the conduits to the manifold where they usually co-mingle before flowing along a single main pipeline to the production platform. The quality and quantity of the fluids produced from each well may vary; for example, one wellbore may produce production fluids that are rich in crude oil and relatively free from produced water and corrosive gasses such as H2S, whereas a neighbouring well drilled to a different depth in the same formation may produce more water, or may have a high content of noxious and corrosive gasses; such a well would be less economically productive and may have higher maintenance costs. Furthermore, different wells tied back to the same manifold may be owned and/or operated by different operators. It is therefore useful to know the quantity and quality of wellbore fluids that are produced from each respective wellbore before they are mixed in the manifold or main pipeline leading from the manifold to the production platform, so that the relative benefits and liabilities of the respective wells can be calculated.
Traditionally this has been done by sampling the fluids produced at each respective wellbore by providing separate sampling conduits or lines that run parallel to (and usually along the outside of) the conduits between the respective wellbores and the manifold, and from there along the main pipeline back to the production platform, where they can be analysed and graded. Separate sampling lines are of course needed for each wellbore, and this causes several problems in that the additional small bore lines often become blocked by viscous fluids and cuttings, or damaged by corrosive agents like H2S, and to address this several lines are normally installed for each wellhead all the way back to the platform, so that backup lines can be brought into operation if the main sampling line for a particular wellhead fails or becomes blocked. This is very expensive and the infrastructure of the extra lines needs to be installed at the beginning of the life of a well, but is seen as the only solution to the problems of being able to sample continuously throughout the life of the well.
According to the present invention there is provided a method for sampling a fluid from a wellbore, the method comprising
a vehicle having a drive means for moving the vehicle, a collecting device for collecting a sample of the fluid and a storage facility for the collected fluid;
using the collecting device to recover a sample of the fluid to the vehicle""s storage facility at a first location on a subsea structure;
storing the sample in the storage facility of the vehicle; and
carrying the sample in the vehicle""s storage facility to a second location.
The present invention also provides a sampling device for collecting samples of fluid produced from a subsea wellbore, the sampling device having;
a drive means for moving the sampling device, a collection device for collecting a sample of fluid and a storage container for holding the collected fluid.
The first location is typically a wellhead but can be other positions of a well such as a wellbore, pipeline from the wellhead, side-track manifold, or main pipeline, storage tank or gravity base structure. The first position typically has a collection port to mate with the collection apparatus. The second position can be onshore, underwater or on a platform or ship such as a remotely operated vehicle or xe2x80x9cROVxe2x80x9d.
Preferably the vehicle is an ROV. Preferably the storage tank and collection device are housed on a frame or skid attached to the ROV. Typically the collecting device comprises at least one sampling bottle, but two or more can be provided.
Typically the vehicle is adapted to interface with the wellhead at the first position, and can be provided with a collecting and sampling probe for insertion into e.g. an aperture on the wellhead. The probe can be connected to the storage tanks or bottles etc by means of conduits. Typically production fluids are extracted via the male/female connection between the probe and the aperture. The collecting device can be arranged to collect and discard a portion of the fluids being sampled, and typically recovers an initial sample of fluid from the collection port of the wellhead to a first sampling bottle. This is done because the fluid lying in the collection port of the wellhead may be static and may not represent a true sample of the fluid flowing through the wellhead. Therefore, the first sample of fluid from the collection port of the wellhead is drawn off to a first sampling bottle and can be kept separate from later samples. Any number of later samples e.g. 3-10 can be taken from the fluid flowing through the wellhead, depending on the number of sampling bottles or partitions in the collection tank that are available. Typically a waste tank is provided at the second position into which the initial samples of static fluid can be discarded.
Typically the vehicle has an array of valves which can be activated independently of each other. Typically different configuration of the valves will direct liquid into each sampling bottle as required. Typically the sampling bottles each contain a piston. Normally a pressure gauge is connected to each sampling bottle. Normally a piston indicator is provided so the position of the piston can be determined from a remote position outside the bottle. Typically the piston indicator moves with the piston, but it may be an electronic indicator that is monitored elsewhere on the ROV or remote from it. Typically the piston indicator is a rod which extends from the piston outside the bottle. Typically the sampling bottles are connected to the male connector via a hose of e.g. {fraction (1/4+L )}xe2x80x3 diameter. The sampling bottles, valves and hose are typically designed to operate in pressure of up to 230 barg and at temperatures of xe2x88x9250 to 130xc2x0 C.
During transportation of the ROV prior to use, the sampling apparatus is typically filled with a liquid. Preferably the liquid is bio-degradable. Typically the liquid is a mixture of water and glycol.
Typically the liquid is contained in the sampling bottles when the vehicle dives. Typically the liquid is expelled from a second end of the sampling apparatus as fluid is recovered from the wellhead. Preferably a control means such as a throttle is provided on the second end of the sampling apparatus to control the rate of expulsion of the liquid. This typically controls the rate of introduction of the fluid from the wellhead into the first end of the sampling apparatus, typically via the piston.
When production fluids have been extracted from the first wellhead the vehicle typically disengages the probe from the collection port on the wellhead and moves (in ROV terminology it xe2x80x9cfliesxe2x80x9d) to the second position. The second position can typically be an offshore platform, a ship or an inshore facility. The vehicle typically docks at the second position where the sample(s) collected may be removed by e.g. removing the sampling bottles and replacing them with empty bottles. Typically only the second and subsequent sampling bottles are replaced. Typically the fluid(s) contained in the second or further sampling bottle is analysed for fluid chemistry.
In several important embodiments of the invention the collection device has several separate containers such as bottles for collecting samples and the vehicle flies between adjacent wellheads to collect samples from each of them before returning to the ship or platform etc for analysis of the samples. In this embodiment the vehicle can collect different samples from adjacent wellheads on a single trip.
While the ROV can be typically tied back to a ship or platform by a conventional umbilical the vehicle need not be a conventional ROV.
Typically the fluids contained in the first sampling bottle will be released into the waste tank. Normally this operation is performed at the second position. Typically a particular combination of open and closed valves can be used to direct fluids from any sampling bottle to the waste tank.
The pipework is typically vented before the sampling bottles are removed. This typically allows the pressure in the pipework to be equilibrate with ambient pressure and so ease the removal of the sampling bottles from the vehicle. Typically a particular configuration of the valves can be used to vent the pipework. After the pipework has been vented the sampling bottle(s) may be removed.
A new bottle is typically attached to the sampling apparatus. Typically the sampling apparatus is flushed with de-mineralised water to prevent cross-contamination between samples. Typically the sampling apparatus is purged with nitrogen prior to a subsequent sampling run in order to remove air from the pipework. Typically the sampling apparatus will be tested for leaks whenever a sampling bottle has been replaced.
Typically the vehicle (or at least the collection device) will undergo a hydrotest before a second operation. In the hydrotest the sampling bottles are filled with de-mineralised water and pressurised up to 230 barg. If no leaks or change in pressure are observed after a period in the order of 30 minutes the vehicle is typically subjected to a gas test. During the gas test, the vehicle (or at least the collection device) is typically submerged in a water bath and is flushed with Nitrogen gas through the probe to subject the sampling apparatus to a pressure of up to 125 barg. Any leaks would clearly be observed in the form of bubbles escaping from the vehicle or collection device.